Hydraulic fracturing is a way of increasing the productivity of wells in the extraction of oil or gas. It involves injection of fluids into an oil-bearing or gas-bearing subterranean formation at sufficiently high rates and pressures to form cracks in the formation that increase the flow of fluids from the oil or gas reservoir into the well.
To maintain cracks in the open state, mechanically strong proppants, which do not react with the borehole fluid, are injected into them. These propping agents are spherical granules (proppants), which penetrate the fracture with the fluid and, at least partially, fill it creating a strong propping frame, permeable for oil and gas being released from the formation.
In general, the proppant is a solid material designed to maintain induced hydraulic fracture in its open state during or after the process of hydraulic fracturing. For hydraulic fracturing to be carried out, proppants are added to the well treatment fluids, which are then being inserted into the subterranean formations. Well treatment fluids can vary in composition depending on a type of formation. Traditional proppants include materials such as sand (the most common type), nutshells, aluminum and aluminum alloys, crushed charred coal, granulated slag, coal dust, crushed stone, granules of metal such as steel, sintered bauxite, sintered alumina, refractory materials, such as mullite and glass granules, as well as artificial ceramic materials and polymers.
The importance of selecting a material, which is suitable for a particular well, is due to the fact that the proppants should resist not only a high reservoir pressure tending to deform the proppant particles, which may lead to the crack closure, but also bear the influence of the aggressive wellbore media (moisture, acid gases, saline solutions) at high temperatures.
It was discovered that ceramic proppants generally have advantageous characteristics with respect to many other types of materials, for example in relation to their durability and uniformity in size and shape.
However, while the ceramic proppants are sufficiently durable and effective and can be produced in economically efficient ways, it is necessary to create new proppants having improved mechanical characteristics such as durability, permeability, specific weight (bulk density), hydrothermal stability and acid resistance, as well as effective methods for producing them.
There are technical decisions for producing proppants, namely, the propping agent (U.S. Pat. No. 5,188,175), which is represented by ceramic granules of a spherical shape made of sintered kaolin clay containing oxides of aluminum, silicon, iron and titanium, where the oxides in these granules are present in the following ratios, wt %:aluminum oxide—25-40, silicon oxide—50-65, iron oxide—1.6 and titanium oxide—2.6. However, this proppant has insufficient durability and is only intended for intermediate depth wells with a pressure of less than 8000 psi.
In addition, a method for producing ceramic proppants made of magnesium-silicate material with a forsterite content of 55 to 80% wt/wt is known from the Russian Federation Patent No. 2235703 C1. According to this process, the original ceramic material based on forsterite is being grinded, granulated and fired at a temperature of 1150 to 1350° C.
The disadvantage of the known method is that under hydrothermal conditions, forsterite becomes partially hydrated, therefore the mechanical durability of the proppant granules reduces significantly.
In the Russian Federation Patent No. 2235702 C2, a similar process is shown wherein the magnesium-silicate precursor consists of magnesium metasilicate with about 40% wt/wt of MgO and about 60% wt/wt of SiO2. Due to the very narrow sintering range (ΔTmax from 10 to 20° C.), the production of such proppants is difficult and expensive. In addition, due to the narrow range of sintering temperature, firing in a rotary kiln under standard industrial conditions will result in underburned porous proppant particles and overburned fused proppant particles.
Thus, the actually achieved durability, acid resistance and hydrothermal stability of the propping agents obtained under industrial conditions are noticeably lower than those for batches obtained under laboratory conditions. Moreover, the narrow sintering range requires a greater curing time for the proppant material at the sintering temperature to achieve a uniform temperature distribution. This leads to the growth of magnesium metasilicate crystals and phase transformation during the cooling process, which also reduces the quality of the resulting proppant.
Thus, the disadvantage of the known method and the product obtained therefrom is that the resulting proppant has got the reduced mechanical characteristics, in particular durability values, which also leads to a decrease in permeability of the proppant layer at elevated pressures.
The object of the present invention is to provide the ceramic proppant (proppant particles) with high operating performance characteristics and low production cost.
In particular, the object of the present invention is to provide a new proppant with improved properties and an economical and energy-efficient method for producing a ceramic proppant, which allows to obtain a proppant with increased durability, reduced bulk density, good permeability, hydrothermal stability, and acid resistance.